1. Field of the Invention
This invention relates to ring laser gyroscope output optics detection systems, and more particularly, it relates to an output optics system for a multioscillator sensor which provides electronic separation of heterodyned Faraday bias difference frequency signals without the need for complex optical signal processing and components.
2. Description of the Related Art
The Ring Laser Gyroscope has been developed as a logical replacement for the mechanical inertial gyroscope. Based upon the principles of the Sagnac Effect, ideally the ring laser gyroscope has minimal moving parts allowing extremely accurate rotational sensing. As originally envisioned, the ring laser gyroscope has at least two counter-propagating electromagnetic waves (such as light) which oscillate within an optical ring cavity. When the ideal ring laser gyroscope is stationary, no rotation is indicated by the sensor. As the ring cavity of the laser gyroscope is rotated about its central axis, the counter-propagating waves interact so that a beat frequency is developed. A linear relationship between the beat frequency and the rotation rate of the gyroscope with respect to the inertial frame of reference may be established.
Although the ideal ring laser gyroscope is characterized by a beat note proportional to the rotational rate, the two mode planar ring laser gyroscope requires rate biasing or mechanical dithering to prevent counter propagating waves from locking at low rotation rates. In an effort to solve this lock-in problem, non-planar multioscillator ring laser gyroscopes have been developed, having more than one pair of counter propagating modes. Briefly, the basic multi-oscillator ring laser gyroscope operates with left circularly polarized (LCP) and right circularly polarized (RCP) light beams and uses a Faraday effect glass device within the cavity or magnetic field on the gain plasma to provide a phase shift between the counter-propagating waves to prevent mode locking. An example of this theory of multioscillator ring laser gyroscope may be found in U.S. Pat. No. 4,818,087 entitled "ORTHOHEDRAL RING LASER GYRO" issued Apr. 4, 1989 to Raytheon Corporation (Terry A. Dorschner, inventor); and U.S. Pat. No. 4,813,774 entitled "SKEWED RHOMBUS RING LASER GYRO" issued Mar. 21, 1989 to Raytheon Corporation (Terry A. Dorschner, et. al., inventor).
In either of these laser gyroscope systems, it is necessary to extract a portion of each beam propagating within the laser cavity to produce two output signals, each one of which represents the difference in frequency between wave pairs having the same sense of polarization within the cavity. For example, in planar ring laser gyroscope systems, rotational information is obtained by monitoring the oppositely directed waves. In the ideal case of a uniformly rotating laser, the frequencies of the waves are slightly different.
The planar gyroscope has a device for combining its oppositely directed beams to obtain a read out which includes a dielectric mirror mounted on one side to the ring laser gyroscope body. Mounted to the opposite surface of the mirror, a prism assembly (which preferably may be an upright symmetric prism) is used to form a fringe pattern. The prism is directly mounted to the mirror to minimize vibrations.
In the planar gyroscope output optics, the fringes are a measure of the instantaneous phase difference between the oppositely directed beams. For the case when the intensities are matched and counter propagating beams are nearly collinear, the fringe pattern is stationary. When the laser gyroscope is rotated, the fringe pattern moves at the beat frequency rate. If the fringe spacing is considerably larger than the dimensions of a photodetector, a measurement of the rotation rate can be made by simply recording the rate at which the intensity maximum moves past the detectors.
The direction in which the fringe pattern moves past the detectors determines the sense of rotation. By using two detectors spaced at 90.degree., or a quarter fringe apart, and a logic circuit, both positive and negative counts can be accumulated to give rotation rate and sense. It should be noted that with this type of readout, the laser gyroscope is inherently an integrating rate gyroscope with a digital output. Thus, with up-down counting, the net number of accumulated counts depends only on the net angle through which the ideal gyroscope is rotated. One complete revolution of a typical gyroscope would produce on the order of 10.sup.6 counts. In summary, the output optics detection system for the planar ring laser gyroscope is relatively straight forward.
The same cannot be said for the multioscillator ring laser gyroscope. Multimode ring laser gyroscopes as known in the art may employ optical crystals and Faraday effect devices to shift the frequency of the laser beams. Heretofore, the biasing and detection schemes which have been proposed have been unduly complex and have had high noise levels associated with them. This was acknowledged as early as 1977 in U.S. Pat. No. 4,123,162 issued to Sanders and assigned to the common assignee of this application. In order to solve the problem of biasing and detecting output signals from a multioscillator ring laser gyroscope, the Sanders '162 patent was directed to a scheme of rotation direction determination through a circuit which dithered the laser plasma current, and used the AC component from the plasma power supply as a phase standard for detecting the sign or direction of rotation of the ring laser gyroscope. Sanders '162 superimposes a differential AC dithering voltage onto the DC voltage of the plasma power supply. A phase reference voltage is synchronized with the AC dither of the plasma and is applied to the synchronous demodulator 78 of Sanders '162. A slight change in the plasma current reduces one beat frequency (characteristic of one gyroscope contained within the multioscillator) and increases another beat frequency. The Sanders circuitry determines the direction of rotation by determining whether the signal is in phase or out of phase with the phase reference signal. Sanders '162 uses a single photodetector to achieve its rotation rate and rotation sense measurements. Sanders '162 also discloses a maximum intensity seeking path length control servo which is not easily adaptable into most multioscillators used due to the complex nature of the intensity curves exhibited in such a scheme.
Another scheme for rotation rate and rotational direction sensing is disclosed in the following U.S. Pat. Nos.: 4,415,266; 4,429,997; and, 4,449,824, all issued to Matthews, Patent '266 and patent '997 are directed to a phase-locked loop system for a multioscillator ring laser gyroscope, while the '824 patent is directed to the structure of the output optics. A complex output optics detector prism structure is disclosed by Matthews, which includes three mirrors (22, 40, and 41), a beamsplitter (42), a set of quarter-wave plates (43 and 53), a set of polarizers (44 and 54), and a set of detector diodes (45 and 55) (as shown in FIG. 2 of the U.S. Pat. No. 4,449,824 ). The electronic signal processing systems disclosed in the '266 and '997 patents are used to process the heterodyned optical output signals provided by complex optics as discussed in the '824 patent. The Matthews' patents are all directed to an overall system which requires complex optics to separate the Sagnac effect modulated Faraday frequencies. Matthews employs a path length control system which compares the optical intensity of the separated signals to produce a path length control error signal. The problems which arise when using complex output optics (besides the difficulty of manufacturing a bulky mechanical structure and optical alignment) include severe optical signal attenuation, and measurement accuracy problems associated with optical signal backscatter. It therefore is desirable to provide an output optics structure and system which is free from the confinement of complex optical signal processing.
One attempt to simplify the output optics system is disclosed is U.S. Pat. No. 4,836,675, issued Jun. 6, 1989 (Martin and Hendow, inventors) and assigned to the common assignee of this application. In this case, the applicants used straight forward optics (similar to the output optics used in a dithered planar ring laser gyroscope system) and rather complex electronics to achieve the goal of measuring rotation rate and sense, as well as achieving cavity length control, in a multioscillator ring laser gyroscope system. The system that U.S. Pat. No. 4,836,675 discloses for cavity length control attempts to discriminate the amount of envelope modulation depth to determine the gyroscope's operating point, using no additional photodetectors than what is required for a planar gyroscope; however, the proposed electronic system for processing the optical output signals are rather complex, and therefore subject to signal degradation and noise, as well as higher cost implementation.